Securities - Distribution of maturities of debt securities with approximate yield of portfolio (Detail) $ in Millions	Sep. 30, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 4,356
Available-for-sale yield within one year	0.35%
Available-for-sale amount after one but within five years	$ 6,335
Available-for-sale yield after one but within five years	1.55%
Available-for-sale amount after five but within ten years	$ 9,853
Available-for-sale yield after five but within ten years	1.77%
Available-for-sale amount after ten years	$ 19,657
Available-for-sale yield after ten years	2.06%
Total fair value within one year	$ 4,362
Total fair value after one but within five years	6,478
Total fair value after five but within ten years	10,425
Total fair value after ten years	20,063
Held-to-maturity amount within one year	$ 59
Held-to-maturity yield within one year	2.72%
Held-to-maturity amount after one but within five years	$ 377
Held-to-maturity yield after one but within five years	2.70%
Held-to-maturity after five but within ten years	$ 499
Held-to-maturity yield after five but within ten years	2.34%
Held-to-maturity after ten years	$ 9,291
Held-to-maturity yield after ten years	2.68%
Total fair value within one year	$ 59
Total fair value after one but within five years	390
Total fair value after five but within ten years	514
Total fair value after ten years	9,681
U.S. Treasury [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 1,123
Available-for-sale yield within one year	0.44%
Available-for-sale amount after one but within five years	$ 3,824
Available-for-sale yield after one but within five years	1.75%
Available-for-sale amount after five but within ten years	$ 6,546
Available-for-sale yield after five but within ten years	1.40%
Available-for-sale amount after ten years	$ 5,236
Available-for-sale yield after ten years	1.87%
US Government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 30
Available-for-sale yield within one year	4.48%
Available-for-sale amount after one but within five years	$ 762
Available-for-sale yield after one but within five years	1.77%
Available-for-sale amount after five but within ten years	$ 3,244
Available-for-sale yield after five but within ten years	2.45%
Available-for-sale amount after ten years	$ 3,672
Available-for-sale yield after ten years	1.94%
Held-to-maturity amount within one year	$ 59
Held-to-maturity yield within one year	2.72%
Held-to-maturity amount after one but within five years	$ 360
Held-to-maturity yield after one but within five years	2.64%
Held-to-maturity after five but within ten years	$ 481
Held-to-maturity yield after five but within ten years	2.25%
Held-to-maturity after ten years	$ 1,295
Held-to-maturity yield after ten years	3.16%
US Government agency issued or guaranteed [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 0
Available-for-sale yield within one year	0.00%
Available-for-sale amount after one but within five years	$ 98
Available-for-sale yield after one but within five years	1.82%
Available-for-sale amount after five but within ten years	$ 1
Available-for-sale yield after five but within ten years	7.87%
Available-for-sale amount after ten years	$ 10,667
Available-for-sale yield after ten years	2.19%
Held-to-maturity amount within one year	$ 0
Held-to-maturity yield within one year	0.00%
Held-to-maturity amount after one but within five years	$ 11
Held-to-maturity yield after one but within five years	3.97%
Held-to-maturity after five but within ten years	$ 14
Held-to-maturity yield after five but within ten years	4.67%
Held-to-maturity after ten years	$ 7,994
Held-to-maturity yield after ten years	2.60%
Obligations of U.S. states and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Held-to-maturity amount within one year	$ 0
Held-to-maturity yield within one year	0.00%
Held-to-maturity amount after one but within five years	$ 6
Held-to-maturity yield after one but within five years	3.59%
Held-to-maturity after five but within ten years	$ 4
Held-to-maturity yield after five but within ten years	4.49%
Held-to-maturity after ten years	$ 0
Held-to-maturity yield after ten years	0.00%
Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 0
Available-for-sale yield within one year	0.00%
Available-for-sale amount after one but within five years	$ 0
Available-for-sale yield after one but within five years	0.00%
Available-for-sale amount after five but within ten years	$ 62
Available-for-sale yield after five but within ten years	4.83%
Available-for-sale amount after ten years	$ 82
Available-for-sale yield after ten years	3.11%
Held-to-maturity amount within one year	$ 0
Held-to-maturity yield within one year	0.00%
Held-to-maturity amount after one but within five years	$ 0
Held-to-maturity yield after one but within five years	0.00%
Held-to-maturity after five but within ten years	$ 0
Held-to-maturity yield after five but within ten years	0.00%
Held-to-maturity after ten years	$ 2
Held-to-maturity yield after ten years	7.01%
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale amount within one year	$ 3,203
Available-for-sale yield within one year	0.29%
Available-for-sale amount after one but within five years	$ 1,651
Available-for-sale yield after one but within five years	0.95%
Available-for-sale amount after five but within ten years	$ 0
Available-for-sale yield after five but within ten years	0.00%
Available-for-sale amount after ten years	$ 0
Available-for-sale yield after ten years	0.00%